Clinical Monitoring Physics: Invasive Pressures, Pulse Oximetry, and Gas Analysis — WACS Viva & Clinical Scenarios (Physics in Anaesthesia)

Exam-style clinical monitoring physics: invasive pressures, pulse oximetry, and gas analysis viva scenarios with examiner probes and model answers for…

Scenarios covered

• SCENARIO 1: A 64-year-old male undergoing coronary artery bypass grafting has a left radial arterial line connected to a disposable transducer. The monitor displays an arterial pressure of 185/55 mmHg with a prominent dicrotic spike and ringing oscillations following the fast-flush test. However, a manual cuff measurement on the contralateral arm reads 145/75 mmHg. Using physical principles, define natural frequency and damping coefficient, diagnose the circuit malfunction, explain why systolic pressure is falsely elevated while diastolic pressure is falsely low, and outline how to correct the system.
• SCENARIO 2: A 24-year-old female with severe burns is receiving intravenous silver nitrate and local prilocaine dressings. During surgery, her arterial blood appears chocolate-brown, and the pulse oximeter reads 85% SpO₂ continuously, failing to rise despite 100% inspired oxygen delivery. Arterial blood gas co-oximetry reveals a methaemoglobin level of 34%. Explain the physical principles of dual-wavelength pulse oximetry based on the Beer-Lambert law, and explain why methaemoglobinaemia forces the pulse oximeter reading to asymptotically fixate near 85%.
• SCENARIO 3: During one-lung ventilation for an elective thoracotomy, you observe the capnography monitor. Explain the physical mechanism of Non-Dispersive Infrared (NDIR) spectrometry for carbon dioxide measurement, differentiate the physical advantages and disadvantages of mainstream versus sidestream gas analyzers, and explain how high concentrations of nitrous oxide or oxygen cause collision (pressure) broadening errors.
• SCENARIO 4: An anaesthetic machine's internal oxygen monitoring system fails to calibrate to 21% room air, reading 14% instead. Describe the physical and chemical principles of a galvanic (fuel cell) oxygen sensor versus a modern paramagnetic oxygen analyzer, explain why galvanic cells deplete over time even when the machine is switched off, and compare their response times.